The present invention relates to an improvement of vinylidene chloride copolymer film. More particularly, it relates to a film having remarkably improved inflation-stretching processability and cold resistance which is prepared by combining a miscible type elastomer and a small amount of a particle dispersion type elastomer with vinylidene chloride copolymer.
The vinylidene chloride copolymer has been widely used for preparing package film and container for food, because of excellent characteristics such as excellent heat shrinkage, low gas permeability, transparency, chemical stability, flameproofing property and heat-sealability of the product thereof.
However, when vinylidene chloride copolymer is used for preparing package film or container, the cold resistance of the product which is one of important requisite characteristics is inadequate, so that the product is easily broken in a package of frozen food or during handling in a frozen place. Accordingly, polyolefin products having high gas permeability or expensive laminated products have been used.
In order to improve the strength of a single layer product at low temperature, it has been known to add a plasticizer, or to add an internal plasticizer or an elastomer etc..
In the addition of the plasticizer, the miscible content of the plasticizer is limited. When the content of the plasticizer is more than 10 wt. %, the migration of the plasticizer on the surface of the product is excessive, so that the appearance is inferior and the low gas permeability of the product is lost, making it unsatisfactory for storage of food.
Moreover, the plasticizer is easily migrated into the food and accordingly, an addition of a large amount of the plasticizer is not preferable from the viewpoint of hygiene. The improvement of the impact strength at low temperature with the plasticizer has not been satisfactory.
The internal plasticization highly affects the crystallinity of vinylidene chloride copolymer which is one of the important characteristics whereby the low gas permeability is deteriorated to a greater degree than by use of a plasticizer. Accordingly, the internal plasticization has not been satisfactory.
The addition of an elastomer causes low miscibility to deteriorate the transparency and only few kinds of elastomers can be practically used.
These elastomers include miscible elastomers having high miscibility such as copolymers of an olefin and a vinyl ester, a vinyl carboxylic acid or a vinyl carboxylic acid ester, or particle dispersion type elastomers having high miscibility on the surface which are produced by a polymerization of a monomer adsorbed on elastomer particles to form the miscible type elastomer coating.
These elastomers have been respectively added to vinylidene chloride copolymer as the method of improving the cold resistance in a molding method having no requisite of severe stretch processability such as a preparation of a tubular container molded by a mold or a sheet processed by a T die.
The particle dispersion type elastomer has been used for providing high elasticity and improving impact strength at low temperature to obtain a rigid container. The miscible type elastomer has been used for improving softness at low temperature to obtain a flexible sheet.
In order to prepare the object product of flexible film, it is preferable to use the miscible type elastomer. However, the content of the miscible type elastomer is remarkably limited in the inflation-stretching process for using the vinylidene chloride copolymer because the precise stretching property is required. Accordingly, it is difficult to attain satisfactory cold resistance such as high flexibility and high impact strength at low temperature.
Thus, the vinylidene chloride copolymer can be biaxially stretched by continuously forming a stable bubble by feeding compressed air at room temperature in a tubular parison prepared by melt-extruding it by a screw type extruder and immediately quenching it to maintain the amorphous condition.
In order to continuously carry out the inflation-stretching process, it is necessary to maintain low inner pressure in the bubble in order for it to be easily expandable and to have enough strength to overcome the inner pressure to be fixed by the crystallization at the stretch orientation.
When the miscible type elastomer is added to the vinylidene chloride copolymer, the dispersion unit is usually less than 500 A to be highly miscible and the glass transition temperature is lowered by the plasticization.
Accordingly, the film can be flexible, however the breaking strength is lowered while the creep deformation and the elongation are highly increased whereby the shape fixing by the stretch orientation is not satisfactory and the bubble expansion is excessive and the film may be broken.
The bubble may be instable when more than 10 wt. % of the miscible type elastomer is added. The bubble formation may be difficult when more than 20 wt. % of the miscible type elastomer is added whereby it is difficult to give satisfactory cold resistance because of the limitation of the content of the miscible type elastomer in the shaping process.